In recent years, tightening of the effluent regulations and increasing community concerns have provided the motivation for further reductions in contaminants, especially the toxic organic chlorinated compounds such as those found in the effluents of pulp and paper mills. As a result, some mechanical process pulp mills are employing zero-discharge technology, while paper mills in Sweden have switched their bleach technology from chlorine-based to a total chlorine-free (TCF) processes.
In the United States, however, most pulp mills are utilizing the Kraft process, which results in effluent volumes being several times higher. In addition, due to the stronger market demand for the high quality paper from the elemental chlorine-free (ECF) process and its lower cost, most of the mills in the U.S. prefer ECF over TCF and a minimum-discharge over a zero-discharge process.
Efforts have been made to improve the ClO.sub.2 bleaching sequence and have succeeded in reducing the TCDD (2, 3, 7, 8-tetrachloro-dibenzo-p-dioxin) in the effluents below its detectable limit (10 parts per quadrillion) and the AOX (adsorbable organic halogens) from 1.5 to 0.3 kg/ton pulp. However, the effect of these reductions on the environment remains uncertain because of the wide differences in toxicity in the hundreds AOX compounds and the extreme toxicity of dioxin. New EPA regulations of 0.272 kg/ton pulp on AOX that will be in effect in 1998, the expected imposition of new and tighter regulations requiring 0.05 kg/ton in fifteen years for AOX in the ECF process due to new toxicological findings, and the competition from the TCF process, necessitates significant reductions of AOX, of specific chlorophenols, and of furans in the effluents from mills employing ClO.sub.2 bleaching.
The traditional technology of wastewater treatment involves evaporation for volume reduction and subsequent biotreatment with a recovery boiler for the destruction of the contaminants. However, this technology is not very efficient for the AOX reduction. Actually, the achieved reduction of AOX to 0.3 kg/ton pulp by the improvement of ECF sequence is close to the limit of the process capability.
AOX is a wide spectrum of organic chlorinated compounds. Many of the compounds are either poorly- or non-biodegradable. The efficiency of AOX removal by biotreatment was found to be 40-50% for effluents from a ClO.sub.2 bleaching unit of a mill. There are reports of 23 to 70% removal of AOX by biotreatment from bleach effluents of mills. It should be also noted that many of these compounds are volatile. In a distillation, AOX will leave with the distillate even if only 10 to 20% is evaporated in a batch experiment. Actually when 90% of the effluent volume is distilled, only half of the AOX remained in the still. Continuous distillation experiments showed that only a third of AOX remained in the bottom product. The actual AOX concentration in distillates from 0-10% up to 80-90% of the effluent volume remained close to a constant value. These results predicted the poor separation of distillate from AOX in the evaporators, which concentrate the bleaching effluent from 0.2-0.3% to 15% by solute weight, and the concentrators, which concentrate it from 15% to 45%.
In other industries, freeze concentration has been used to effectively keep the volatile solute in the concentrate. The concentration of aqueous solutions by freezing the solution and removing the resulting ice has been studied extensively. Generally, however, freeze concentration of fruit juices, like orange and apple juice, and of coffee are the only commercialized industrial processes. These are relatively small volume processes.
The application of freeze concentration to large volume processes such as the desalination of sea water or the waste water or toxic effluents treatment is hindered by technical problems such as the formation of ice scaling on the cooling surface and the plugging of the tubes of the heat exchanger, entrapment of concentrate in the produced ice, and the high refrigeration cost.
The NIRO process, used currently for fruit juice concentration, has solved the technical problems by decoupling the nucleation and the growth processes for ice crystals and controlling them separately. The formation of ice nuclei (nucleation) takes place on the cooling surface of an expensive scraped-surface heat exchanger (VOTATOR). The ice nuclei are then transferred to a crystallizer where they grow.